This invention relates to circuits for starting and ballasting gas discharge lamps. More specifically, this invention relates to inverter circuits for operating gas discharge lamps in resonant circuits wherein the inverter switching is controlled as a function of output current.
Starting and ballasting circuits are generally required for the stable and efficient operation of gas discharge lamps. Under operating conditions, the discharge lamp appears as a negative impedance device; that is, the voltage drop across a discharge lamp will tend to decrease with increasing discharge current. Ballasting circuits are normally connected to provide a positive series impedance or other current limiting mechanism to balance the negative lamp characteristic and provide stable operation. The voltage required to initiate a discharge in such lamps is, generally, substantially higher than the normal operating voltage of those lamps. Auxiliary starting circuits are generally utilized to provide a high starting voltage for a relatively short time to initiate a discharge.
In the prior art, the lamp ballasting function has generally been provided by an inductor or resistor connected in series with the discharge. The lamp starting the function has been provided by a number of circuits comprising for example, saturable magnetic cores or inductive voltage generators which were activated by time delay switches.
U.S. Pat. No. 3,781,638 and my copending U.S. patent application Ser. No. 516,722, now Pat. No. 3,953,780, with John P. Walden describe a class of solid state ac inverter circuits wherein commutation is initiated as the instantaneous load current crosses a predetermined threshold. The operating frequency of such circuits, therefore, tends to increase as the load impedance decreases. I have determined that these inverter circuits may be combined with gas discharge lamps connected in a resonant circuit to provide ballasting and starting functions.